Assuming that the problem solution implementation is a technology based solution, I would identify the following as some of the core keys to success:
1) Involve the right people
An implementation is only as successful as the end users deem it to be. So involve them! Subject matter experts from the business should be involved from the start, from documenting needs and the existing process to UAT and train he trainer sessions.
Additionally project sponsor(s) should be involved at all stages of the process as the project sponsor is the liaison between the executives and the success of the program.
IT, both in-house and from the vendor should obviously be involved at all stages.
2) UAT, UAT, UAT
Testing the implemented process from start to finish is critical to ensuring success of the system. From making sure the system addresses the problems it set out to meet and ensuring system access and security is correct, to ensuring basics like system functionality and links work - UAT is critical to the success of ANY implementation.
Communication with project stakeholders helps to set up expectations and helps to address questions before they become questions. Pre-implementation communication including system functionality, system overviews and anticipated time lines are critical. Post-implementation communications like FAQ's, support logistics, and ongoing updates ensure that users know the support is there from the top down and that the implementation is being driven and supported from the top down.
4) Post Implementation Support
Utilizing the tools in the communication plan is important, but for a large, enterprise level implementation, having a support plan in place is critical. Who will answer questions? How will questions be addressed? Who will provide on the floor support if needed?
While these are just a portion of an overall successful implementation, the caveat is just that...we are at the implementation stage of the process. So many moving parts must be effectively managed and brought together just to get to the implementation stage but once there, keeping the 4 points above in mind will surely help to ensure overall success.